Lone Planets “More Common Than Stars”

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We happen to live in a solar system where everything seems to be tucked neatly in place. Sun, planets, moons, asteroids, comets… all turning and traveling through space in relatively neat and orderly fashions. But that may not always be the case; sometimes planets can get kicked out of their solar systems entirely, banished to roam interstellar space without a sun of their own. And these “orphan planets” may be much more numerous than once thought.

Researchers in a joint Japan-New Zealand study surveyed microlensing events near the central part of our galaxy during 2006 and 2007 and identified up to 10 Jupiter-sized orphan worlds between 10,000 and 20,000 light-years away. Based on the number of planets identified and the area studied they estimate that there could literally be hundreds of billions of these lone planets roaming our galaxy….literally twice as many planets as there are stars.

“Although free-floating planets have been predicted, they finally have been detected, holding major implications for planetary formation and evolution models.”

Previous observations spotted a handful of free-floating, planet-like objects within star-forming clusters, with masses three times that of Jupiter. But scientists suspect the gaseous bodies form more like stars than planets. These small, dim orbs, called brown dwarfs, grow from collapsing balls of gas and dust, but lack the mass to ignite their nuclear fuel and shine with starlight. It is thought the smallest brown dwarfs are approximately the size of large planets.

On the other hand, it is likely that some planets are ejected from their early, turbulent solar systems, due to close gravitational encounters with other planets or stars. Without a star to circle, these planets would move through the galaxy as our sun and other stars do, in stable orbits around the galaxy’s center. The discovery of 10 free-floating Jupiters supports the ejection scenario, though it’s possible both mechanisms are at play.

“If free-floating planets formed like stars, then we would have expected to see only one or two of them in our survey instead of 10. Our results suggest that planetary systems often become unstable, with planets being kicked out from their places of birth.”

– David Bennett, a NASA and National Science Foundation-funded co-author of the study from the University of Notre Dame.

The study wasn’t able to resolve planets smaller than Saturn but it’s believed there are likely many more smaller, Earth-sized worlds than large Jupiter-sized ones.

That discusses a second star in a binary system which would have a better chance of surviving of course but a gas giant may well survive. Supernovae are often asymmetric and the exploding star may be ejected out of the system leaving the planets behind to wander off on their own.

My earlier comment goes along with this; that there is undoubtedly much more “ordinary” mass that we don’t see “yet” that contributes to the CDM thereby lessening the calculated non-baryonic contribution.

What would the speed of those objects be relative to the Sun?
Can we determine a maximum speed?

Most stars including the sun probably moves in the same general direction around the milky way, so estimate that these objects would have a speed a bit higher than escape velocity of their previous star.

The escape velocity would have to be achieved, but many situations can cause this to be achieved. For example a small rocky world careens around a massive jupiter in a near miss, being flung towards the parent star, wrapping around in a near miss, giving it enough momentum to produce a highly elliptical orbit, which that massive jupiter can knock it out of pulling it from the sun. It’s speed could also be lower then the parent star, if a large planet on the outskirts of a planetary disk tugs/speeds up a smaller planet in its wake, flinging it out of the solar system. There are also a few scenarios where the planet can be flung in the “wrong direction”, such as orbiting 180º around a planet before escaping. Orbital movements are stable regardless of direction.

About maximum speed, theoretically there is no limit. The fastest known star moves at 715,214 m/s (0.002c) (compare this to the fastest spacecraft speed to date of 4,000 m/s). Any gravational interaction can accelerate an object in orders of magnitude above terminal velocity. A star can accelerate an object to a maximum of the orbital speed at the tightest possible orbit (remembering that the closer an object is, the faster it needs to go to orbit, or inverted the closer an object is, the faster it will be accelerated). However a tight binary pair can rotate incredibly fast, to a limit of when the sheer speed destroys either of them. Given the theoretical existence of binary black holes, there is no rotation limit.

Voyager 2 is leaving our system at 15.5 km/s. Of course spacecraft deeper in gravity wells go faster, but that’s apples and oranges. I don’t know how much more it will slow down, but pretty sure the ultimate speed is more than 4 km/s.

What would the speed of those objects be relative to the Sun?
Can we determine a maximum speed?

Most stars including the sun probably moves in the same general direction around the milky way, so estimate that these objects would have a speed a bit higher than escape velocity of their previous star.

It would seem that they measured the density along the line of sight to the galactic center, and then extrapolated to the rest of the galaxy. So, the extrapolation process means this prediction says little about the galactic halo where the dark matter is.

I’m not an astronomer, but I gather that planets are disqualified for other reasons, such as insufficient time over the universe’s lifespan to form that much heavy matter.

I have no idea how much metal. But CDM is also an explanation of spiral galaxy structure, and if memory serves the other kind of gravitational lensing has revealed that spiral structure predated the existence of much metal at all.

Also, my reply was intended for wjwbudro, I don’t recall seeing your comment before at all…

Even if there was many more than expected, this would be a tiny drop in the bucket of galactic mass. Remember that a star is huge, 99% of the matter in our solar system is in Sol. Assuming this is not atypical, this means that for all the visible matter in stars, only a tiny fraction can be planets. Currently, it is held that there is more “missing” matter then visible.

If you are meaning how often a rogue planet off alone orbiting the galaxy effects planets in our solar system – almost assuredly never. We’d be far more likely to effected by a nearby star. Think of it this way…for us to notice the effect of gravity from any passing object, it has to get close enough to overpower the gravity from the Sun to at least some degree. The closer to the Sun an object is, the more massive/closer that object would have to be. The areas of the solar system where the known planets reside is but a fraction of the Sun’s gravitationally dominated space. (the Oort Cloud is hypothesized to extend out to 50,000 AU, which is almost a light year).

Take a good-sized trampoline, put someone who weighs 400+ lbs in the middle, and then drop a marble on the edge somewhere. Observe how much of a dent the marble makes. You can even use more massive objects, but keep in mind how close they are getting to the weight of your ‘star’. Now imagine that trampoline was the size of a small town and just think about how big and how close one of those little marbles would have to get to even be noticeable. Not trying to be patronizing, but essentially the likelihood of having to worry about something like that is so negligible (even astronomically speaking) that it might as well just be listed as Zero.

Hey this almost comes back to the comment I made a couple of weeks ago in regards to the planet that was found roating counter to the orbital direction of the other planets in that system. My comment at the time was “Could this planet originally be from outside the system it now inhabits and have been captured by the stars gravity and enter its orbit, albeit in the wrong direction?”.
I can’t remember who answered me but the answer was that the probability was minute but I would have to assume that at least it was possible……..well at least after reading this I’d have to say that. Certainly not impossible.

Not impossible no, and it would up the frequency of capture attempts. But it wouldn’t tip the balance that a retrograde planet most likely is a disturbed original planet, since those are at hand (naively more attempts of disturbance).

It would be similar to my reply above to Potato. The unlikeliness by comparison to it being a disturbed native planet’s orbit is not even close. Taking that bet would be similar to a footrace between an olympic runner and average joe with a broken leg…sure Joe could win…technically…

Depends on what you mean with capture. You can’t capture a rough object without some mechanism that reduces its speed below escape velocity of the big star/planet that wants to capture it. In order to slow down, something else must speed up.

The Bullet Cluster illustrates the nature of /CDM, where the luminous ordinary matter in the galaxy collision interacts by various forces to stick together, while the DM from the two galaxies continues on their original trajectories. Ordinary matter of most any kind would not behave this way.

These lone wolf planets are a very small percentage of a galactic mass. The article here indicates the detection of 10 of these planets by microlensing in a stellar formation region. This does not suggest an overwhelming amount of matter is locked up in these planets.

These planets are most likely very cold jovian planets. Interstellar space is about 30deg-C and that is really pretty cold. I doubt these planets have anything we would call life. There were speculations about life in the atmosphere of Jupiter, floating balloon-like organisms and fliers and so forth. Yet I suspect these ideas amount to wishful speculation. In fact I think these ideas about life in the sub-ice layer regions of Europa and Ganymede might also amount to much the same.

I agree completely on the life statements. I’d place my bets on Titan/Mars/Ceres for some sort of life long before anything else in the solar system. Dawn needs to hurry up with the dead rock with a flat bottom and move on to our little dwarf H2O buddy.

Also the whole subsurface oceans thing…it would require an effort like what they’ve been doing at Lake Vostok and I don’t see that being all that feasible for a space probe project currently.

I had a strong suspicion that there are more rogue planets than stars. In stars, the lower the mass, the more common the star is. Going by this, I figure there are more Brown dwarfs then M-stars, and more Jovian-type orphan/rogue planets than brown dwarfs. I suppose there might be even more smaller planets, including rocky or icy worlds. No doubt the number of comet and asteroid-like objects is even greater.

It’s crowded out there.

The question remains – how many formed in solar systems and ejected, and how many formed independently through gas collapse, not even reaching brown dwarf scale.

Also, how will these be defined (orphaned planets, rogue planets, planemos, etc.)

I posted this earlier and you reaffirmed my thinking;
that there is undoubtedly much more “ordinary” mass that we don’t see “yet” that contribute to the CDM thereby lessening the calculated non-baryonic contribution.

Even if there’s 10 times more free floating planets than stars they would still make up a small fraction of a percent of the galaxy’s mass. The difference would fall well within the error bars for our current estimates of our galaxy’s mass.

Even if there’s 10 times more free floating planets than stars they would still make up a small fraction of a percent of the galaxy’s mass. The difference would fall well within the error bars for our current estimates of our galaxy’s mass.

Even if there’s 10 times more free floating comments than articles they would still make up a small fraction of a percent of the site’s text mass. The difference would fall well within the error bars for our current estimates of our site’s mass. =D